The present invention relates to a layer-built solid state image sensing device. With the spread of home video cameras, higher resolution and higher sensitivity being increasingly required for solid state image sensing devices. In order to improve the resolution, it is necessary to increase the integration rate of pixels which are units for constituting an image, and this inevitably reduces the dimensions of the pixels. However, the quantity of light incident upon one pixel decreases with decreasing pixel dimensions, and therefore the sensitivity thereof is inevitably reduced. As described above, there exists a contradictory relationship between the higher resolution and the higher sensitivity of the solid state image sensing device.
From the standpoint as described above, since the light receiving area of each pixel is required to be increased to the allowable limit of the occupied area, a layer-built solid state image sensing device where an optoelectro transducing layer and a charge transferring layer are arranged in three dimensional manner has been proposed as one effective way of solving the above-mentioned problem.
FIG. 6 shows a cross-sectional view showing the major parts of the prior art layer-built solid state image sensing device. In this prior art layer-built solid state image sensing device, an n.sup.+ -type charge storage layer 5 for storing optoelectro transduced charges and an n-type channel impurity layer 6 for transferring the charges are arranged with gaps therebetween on regions separated column by column by an element isolating layer 8 on the surface of a p-type semiconductor substrate 1. The above-mentioned gaps form a surface transfer channel portion 10 which serves as a transfer route for transferring signal charges from the n.sup.+ -type charge storage layer 5 to the n-type channel impurity layer 6.
Furthermore, a charge transfer electrode 7 is provided over the n-type channel impurity layer 6 and the surface transfer channel portion 10 via an insulating layer 9. The insulating layer 9 further extends over the charge transfer electrode 7. Furthermore, a metallic electrode 4 used in common as a light shading film is provided over the insulating layer 9. The layer-built optoelectro transducing film 2 made of amorphous substance (e.g., amorphous silicon) is formed on the metallic electrode 4. The charge storage layer 5 can be ohmically connected to metal, because of its high impurity concentration. Therefore, the electrode 4 is directly brought into contact with the charge storage layer 5 for electrical connection. Further, a grounded transparent electrode 3 is formed on the surface of the optoelectro transducing film 2 for fixing the surface potential of the optoelectro transducing film 2.
The operation of the prior art layer-built solid state image sensing device as described above will be described hereinbelow. The incidence of light on the optoelectro transducing film 2 produces pairs of electron and hole. The produced electrons are moved and stored in the n-type charge storage layer 5 as a signal, and the produced holes are moved to the transparent electrode 3 and further transferred to the outside therethrough.
FIGS. 7A and 7B show the potential distribution of the n.sup.+ -type charge storage layer 5, the surface transfer channel portion 10, and the n-type channel impurity layer 6. FIG. 7A shows the charge storage status, in which the potential of the transfer channel 10 is approximately equal to that (shown by 13) of the substrate 1, and the signal charge 11 is kept and stored under the n-type charge storage layer 5 in addition to the charge 12. FIG. 7B represents the status where the stored signal charge 11 is transferred from the n.sup.+ -type charge storage layer 5 to the n-type channel impurity layer 6. When a high voltage is applied to the transfer electrode 7, since the surface potential of the transfer channel 10 increases up to a potential indicated by 15 and further the transfer channel potential 14 rises, the stored signal charge 11 is transferred to the transfer channel layer 6 beyond the barrier of the potential 15. In this case, however, the charge stored in the charge storage layer 5 is not necessarily transferred perfectly, but a residual charge 12 remains in proportion to the potential 15.
As described above, in the prior art layer-built solid state image sensing device, the residual charge 12 inevitably remains in the n.sup.+ -type charge storage layer 5, as far as a voltage within a practical range is applied to the transfer electrode 7. Further, since the metallic electrode 4 will not be depleted, in the status as shown in FIG. 7B, a thermal diffusion current due to the residual charge including the charge in the metallic electrode 4 flows beyond the barrier potential, and then is mixed with the signal charge. The above-mentioned phenomenon occurs even in the status where the signal charge is zero, thus causing a problem in that a so-called residual image is inevitably produced.
Furthermore, in the prior art structure, since the barrier potential 15 fluctuates due to thermal and electrical disturbances, the transferred charge also fluctuates, thus causing another problem in that the above-mentioned fluctuations cause noise components in the signal charge and therefore the signal charge is disturbed.
In addition, since a well-known interface level is present in the depletion portion at which the n.sup.+ -type charge storage layer 5 and the element separating layer 8 are in contact with the insulating layer 9, there exists another problem in that a so-called dark current flows as the noise components of the signal.